This invention relates to a method of producing a synchronous belt in which the surfaces of tooth parts are covered with an extensible fabric and a fabric positioning apparatus used in the method, and particularly relates to measures for placing a joint of the extensible fabric formed into a cylindrical form by sewing on the tooth tip face of the tooth part of the synchronous belt.
In a typical synchronous belt used as a power transmission belt for vehicle engine and other applications, a tension member cord is embedded in a belt body along a longitudinal direction of the belt, a multiplicity of tooth parts are formed on the belt body at specific intervals along the longitudinal direction of the belt in a single-piece structure integral with the belt body and the extensible fabric is adhered to the surfaces of the tooth parts.
Such a synchronous belt is generally produced in the following manner. First, an extensible fabric is sewed into a cylindrical form thereby forming a joint, and the extensible fabric thus obtained is fit around a molding drum. The outer periphery of the molding drum has a multiplicity of tooth part forming grooves which are formed at specific intervals in a circumferential direction of the molding drum so as to extend in a direction orthogonal to the circumferential direction. Next, a tension member cord is spirally wound around the extensible fabric and an unvulcanized rubber sheet is then wrapped around the extensible fabric wound with the tension member cord. Thereafter, the unvulcanized rubber sheet is subjected to cure through the application of pressure, so that the tooth part forming grooves are filled with parts of the unvulcanized rubber, respectively, thereby forming a cylindrical slab whose inner periphery has a multiplicity of internal teeth extending in a direction orthogonal to the circumferential direction of the slab. The slab thus obtained is cut in round pieces at specific widths. As a result, obtained are synchronous belts in each of which the tension member cord is embedded in the belt body along the longitudinal direction of the belt, the multiplicity of tooth parts are formed on the belt body at specific intervals in the longitudinal direction of the belt in a single-piece structure integral with the belt body and the extensible fabric is adhered to the surfaces of the tooth parts.
Meanwhile, since the joint of the extensible fabric in cylindrical form is generally formed by sewing with a machine sewing thread, if the joint after the formation of a belt is located at a bottom land or a tooth root, a breakage of the machine sewing thread at the joint may occur due to friction, bending, tension or the like of the joint at the time of engagement with a synchronous pulley thereby opening the joint. This exposes a rubber layer of the tooth part through the opening of the joint and brings the rubber layer into direct contact with the synchronous pulley, resulting in ease of production of a crack. Therefore, to avoid such a situation, it is necessary to correctly position the joint on the tooth tip face of the tooth part.
However, in the above-mentioned method of producing a synchronous belt, even if the extensible fabric is positioned, when fit around the molding drum, so that the joint is located at one of the tooth part forming grooves, the joint moves when the tension member cord is wound after that so that the position of the joint is not fixed. This invites the above-described inconvenience.
As a measure for positioning the joint of the extensible fabric, there is proposed a method as disclosed in Japanese Patent Application Laid-Open Gazette No. 63-69627. This method is described bellow.
First, when the extensible fabric is sewed into a cylindrical form, a solid bar is integrally sewed in the joint. Next, one end of the bar is hitched to a flange provided at one end of the molding drum while the other end thereof is fixed by a tape-like substance at the other end of the molding drum.
Thus, the joint is held in the tooth part forming groove of the molding drum together with the bar and at last the joint can be correctly positioned on the tooth tip face of the tooth part of the synchronous belt. After the subsequent step of winding the tension member cord is completed, the bar is pulled out of between the extensible fabric and the tension member cord.
However, the above method disclosed in the gazette has the following problems:
1. The method requires a special technique for integrally sewing the bar in the joint of the extensible fabric.
2. When a plurality of joints exist in one extensible fabric, much time and effort are expended in sewing bars in respective joints and the necessary number of bars increases.
3. The extensible fabric must be carried in integral form with the bar. This increases the weight and therefore invites inconveniences in transportation and storage.
4. The method requires a bar longer than the axial length of the molding drum. Such a long bar is inconvenient to store.
5. The molding drum must be provided with the flange for fixing one end of the bar. This complicates the molding drum configuration.
6. The bar is difficult to pull out of the tooth part forming groove unless pulled out with a large force, because the bar experiences a tension of the machine sewing thread at the time of integrally sewing the bar in the joint and a force of winding the tension member cord. This is a point to which greatest attention should be paid. This further requires a special tool for pulling out the bar from one end side of the molding drum and also requires to provide, on the one end side of the molding drum, a stroke sufficient for pulling out the bar, that is, a space having at least the length of the bar.
To cope with the sixth problem, it can be considered to use the following method. In this method, the bar is not sewed in the joint of the extensible fabric. Instead of this, the joint is pressed from above by the bar so that the joint is held in the tooth part forming groove together with the bar. Thus, since only a force of winding the tension member cord acts on the bar, the bar can be easily pulled out of the tooth part forming groove.
In this method, however, when both ends of the bar are fixed on both end sides of the molding drum, respectively, a middle portion of the bar is lifted outwardly along a radial direction of the molding drum so that the joint cannot entirely be pressed by the bar. This may cause the joint to move when the fabric is wound with the tension member cord, thereby making it difficult to position the joint. Further, even though a force acting on the bar is only a force of winding the tension member cord, the bar cannot absorb the force of winding the tension member cord because of its solidity as well as be tightly bound by the tension member cord. Therefore, a large force is required for pulling the bar out of the tooth part forming groove of the-molding drum. Thus, also in this method, the bar cannot easily be pulled out as in the case of the above-described gazette. Furthermore, this method also has the problem of requiring a space for pulling out the bar as in the case of the above-described gazette.